Technical Field
This invention relates generally to medical devices and particularly to an endoluminal or subcutaneous graft for improving helical blood flow in a vessel.
Background
A primary physiological function of the aorta and its major branches is to convert the highly pulsative output of the left ventricle to a more nearly uniform and steady flow in the arterioles and capillaries, with minimum loss of energy. This requires that the peripheral vascular input impedance (which is a complex function of arterial resistance, fluid inertance, and arterial compliance) be matched to the output impedance of the heart.
Compliance is the ability of a vessel to distend and increase volume with increasing transmural pressure or to resist recoil towards its original dimensions on application of a distending or compressing force. Compliance can be defined as the fractional change in volume per change in pressure. In compliance, an increase in volume occurs in a vessel when the pressure in that vessel is increased. The tendency of the arteries and veins to stretch in response to pressure has a large effect on perfusion and blood pressure. Blood vessels with a higher compliance deform easier than lower compliance blood vessels under the same pressure and volume conditions. Veins have a much higher compliance than arteries (largely due to their thinner walls).
When a vessel loses compliance, it loses elasticity and typically becomes stiffer. Vessels, such as the aorta, can lose compliance due to age, congestive heart failure, atherosclerosis, etc. As the aorta stiffens and loses compliance, the heart struggles to pump blood and must work harder to eject the same volume of blood from the left ventricle into the aorta with each heartbeat. For example, a young person has a typical compliance of 6% dilation of the aorta with each heartbeat, whereas an older person with some arterial disease has a typical compliance of only 3%. If the heart is incapable of working harder because of underlying diseases, then less blood will be ejected into the aorta with each heartbeat.
A prostheses may be inserted into a body lumen such as an anatomical vessel or duct for various purposes. Prostheses may maintain or restore patency in a formerly blocked or constricted passageway or they may be used for different procedures, such as to facilitate dialysis.
Existing vascular grafts, including stent-grafts, covered stents, arterial bypass grafts, and arterio-venous grafts may be prone to stenosis or neointimal hyperplasia at the ends of the grafts, especially where the venous end of arterio-venous grafts are sutured to the vein. This occurrence has been attributed to a mismatch in the compliance of the graft compared to the vein. The graft may be substantially less compliant than the vein, which may lead to mechanical stresses on the vein and hemodynamic changes.
Additionally, literature has indicated that spiral laminar flow is the native flow regime in large arteries. “Recent work in cardiac and peripheral vascular blood flow has shown evidence for an elegant complexity to flow within the heart and in the large to medium arteries. Blood flow is normally described as laminar in that the blood travels smoothly or in regular paths. The velocity, pressure, and other flow properties at each point in the blood remain constant, all parallel to each other . . . . However, MRI and color Doppler flow imaging techniques have demonstrated that there is a spiral/helical/rotational property to laminar blood flow.” Peter A. Stonebridge, “Three-Dimensional Blood Flow Dynamics: Spiral/Helical Laminar Flow,” MDCVJ (2011).
For example, helical laminar flow has been shown within the aorta and may be a role in maintaining blood flow as the blood passes through the curved aortic arch. Similarly, helical laminar flow has been shown in peripheral and superficial arteries.
Helical flow may have a significant role within the aorta and other vessels. For example, it may have a positive role in the transportation of oxygen to the vessel wall. It may also increase wall shear stress, which results in fewer instances of thrombosis, turbulence, stenosis, and neointimal hyperplasia.
Endoluminal prostheses may be inserted into a body lumen such as an anatomical vessel or duct for various purposes. Prostheses may maintain or restore patency in a formerly blocked or constricted passageway or they may be used for different procedures, such as to facilitate dialysis.
Existing vascular grafts, including stent-grafts, covered stents, arterial bypass grafts, and arterio-venous grafts may be prone to stenosis or neointimal hyperplasia at the distal anastomosis, which eventually can occlude outflow.
It is therefore desirable to have a device configured to induce helical blood flow. As a result, there may be reduced thrombosis within the graft and reduced neointimal hyperplasia and stenosis on the distal end of the graft where it transitions to the blood vessel.